Plate Tectonics Theory – Geography Optional Notes

The technical term for the rigid lithospheric slabs or crustal layers is “plates.” The study of the evolution, nature, and motion of these plates, and the resulting reactions, is known as “plate tectonics.” Essentially, the entire process of plate movements is encompassed by Plate Tectonics. As these individual lithospheric plates move slowly over the surface of the Earth, they slide over the weak asthenosphere, similar to how a pack of ice in the Arctic Ocean drifts under the force of currents and winds.

Introduction to Plate Tectonics Theory

The scientific achievement of the 1960s known as plate tectonic theory is founded on two major scientific concepts: the idea of continental drift and the concept of sea-floor spreading. Essentially, the lithosphere is composed of internal rigid plates. Six major and 20 minor plates have been identified so far (Eurasian plate, Indian-Australian plate, American plate, Pacific Plate, African plate and Antarctic plate)

Historical Background and Development of Plate Tectonics Theory

• The term “plate” was first used in 1965 by Canadian geophysicist J.T. Wilson.
• In 1967, McKenzie and Parker explained the mechanism of plate motion using Euler’s geometric theorem and proposed the “paving stone” hypothesis.
• The “paving stone” hypothesis suggested that the oceanic crust is newly formed at mid-oceanic ridges and destroyed at trenches.
• In the same year, Isacks and Sykes confirmed the “paving stone” hypothesis.
• In 1968, W.J. Morgan and Le Pichon elaborated on various aspects of plate tectonics.
• Plate tectonics has established continental drift and displacement as a reality.

Plate Boundaries and their Classification

Plate boundaries, also known as plate margins, are of utmost significance in tectonic activity. All seismic events, volcanic eruptions, mountain building, faulting, and other phenomena take place along these boundaries. Therefore, studying plate margins in detail is not only desirable but also essential. Typically, plate margins are categorized into three groups:

Constructive Plate Margins

Divergent plate margins, also known as accreting plate margins, are zones of divergence where new oceanic crust is continuously formed due to the continuous upwelling of molten material (lava). These plate margins are also referred to as constructive plate boundaries. At mid-oceanic ridges, the oceanic plates split apart and move in opposite directions.

Destructive Plate Margins

Convergent plate margins, also known as consuming plate margins, occur when two plates move towards each other and converge along a line. At these boundaries, the leading edge of one plate overrides the other plate. The overridden plate is subducted or thrust into the mantle, resulting in a part of the crust (plate) being lost in the mantle.

Conservative Plate Margins

Shear plate margins, also known as conservative plate margins, occur when two plates slide past each other along transform faults. At these boundaries, crust is neither created nor destroyed.

In 1960, H. Hess put forward the concept of plate tectonics to support continental drift. The movement of plates is responsible for the motion of continents and oceans. The present arrangement and shape of continents and ocean basins could have been achieved through the continuous relative movement of different plates of the second Pangaea since the Carboniferous period. Plate tectonic theory is based on evidence from sea-floor spreading and palaeomagnetism.

Sea-Floor Spreading

Introduction to Sea Floor Spreading

• Sea floor spreading first proposed by Harry Hess in 1960.
• Based on research findings of marine geologists, geochemists, and geophysicists.
• Magnetometer used to obtain significant information about magnetism of sea-floor rocks in Pacific Ocean by Mason of Scripps Institute of Oceanography.
• Magnetic anomalies obtained from survey displayed well-defined patterns of stripes.
• Hess proposed mid-oceanic ridges situated on rising thermal convection currents from mantle.
• Oceanic crust moves in opposite directions from mid-oceanic ridges.
• Molten lavas cool down and solidify to form new crust along trailing ends of divergent plates.
• Continuous creation of new crust along mid-oceanic ridges and expanding crusts (plates) destroyed along oceanic trenches
• Continents and ocean basins in constant motion.

Discovery and History of Sea Floor Spreading

In 1963, W.G. Vine and Mattheus conducted a magnetic survey of the central region of the Carlsberg Ridge in the Indian Ocean. They calculated magnetic profiles using general magnetism. However, upon comparing these profiles to magnetic anomalies obtained from the actual survey data, they noticed significant differences.

To address this issue, Vine and Mattheus created magnetic profiles based on alternate bands of normal and reverse magnetism. They divided these profiles into separate stripes, each with a width of 20 km on either side of the ridge. When they compared these new profiles to the observed profiles, they found complete parallelism between the two.

Vine and Mattheus believed that the alternating bands or stripes of magnetic anomalies observed on either side of mid-oceanic ridges were the result of the sea-floor spreading theory proposed by Deitz and Hess. They argued that as molten lava rose with thermal convection currents along the mid-oceanic ridges and cooled, they became magnetized in accordance with the Earth’s magnetic field at the time.

This process divided the pre-existing basaltic layer into two equal halves, which then slid horizontally on either side of the mid-oceanic ridge. The findings of Cox, Doell, Dalrymple, Opdyke, and Heritzler have supported this theory, showing that there is a reversal in the Earth’s main magnetic field and that normal and reverse magnetic anomalies appear in an alternating pattern on either side of mid-oceanic ridges.

Furthermore, there is complete parallelism in the magnetic anomalies on either side of the mid-oceanic ridge, as well as in the time sequence of paleomagnetic epochs and events calculated for the past 4.5 million years based on the magnetism of basaltic and sedimentary rocks.

Figure illustrates the position of magnetic stripes on either side of the mid-oceanic ridge throughout their formation.

Magnetic Stripes and Sea Floor Spreading

• Sea-floor spreading is a continuous process.
• New basaltic crust is constantly formed along mid-oceanic ridges.
• The newly formed basaltic layer is divided into two equal halves and moves away from the mid-oceanic ridge.
• Alternate stripes of positive and negative magnetic anomalies can be observed on either side of the mid-oceanic ridge.
• These magnetic anomalies result from temporal reversals in the Earth’s magnetic field.
• Rocks formed during reverse polarity denote negative magnetic anomalies.

Based on the above facts, the age of magnetic stripes, the rate of sea-floor spreading, and the time of drifting of different continents can be calculated.

Dating of magnetic stripes formed up to 4.5 million years before the present has been completed using information obtained from the survey of paleomagnetism of sea-floors in different oceans.

Mechanisms of Sea Floor Spreading

• The rate of sea-floor spreading is determined based on two factors: age of isochrons and distance between two isochrons.
• The rates of spreading of different oceans have been calculated based on these principles.
• The Atlantic and Indian Oceans are expanding at a slow rate of 1.0 to 1.5 cm per year while the Pacific Ocean is expanding at a rate of 6.0cm per year.
• The rate of sea-floor spreading always means the rate of expansion only on one side of the mid-oceanic ridge.
• Recent studies have shown that the maximum spreading of the Pacific Ocean is 6 to 9 cm per year (total expansion 12 to 18 cm/year) along the eastern Pacific ridge between equator and 30°S latitude.
• The southern Atlantic Ocean is spreading along the southern Atlantic ridge at the rate of 2 cm per year (total expansion 4 cm/year).
• The Indian Ocean is expanding at the rate of 1.5 to 3 cm per year (total expansion being 3 to 6 cm/year).

Plate Tectonics and Continental Displacement

The evidence of palaeomagnetism and sea-floor spreading has validated that continents and ocean basins have never been permanent or stationary throughout the geological history of the earth. Instead, they have always been mobile and are still moving in relation to each other.

Scientists have discovered ample evidence of the opening and closing of ocean basins. For instance, the Mediterranean Sea is the remnant of the once-vast Tethys Sea, and the Pacific Ocean is contracting gradually due to the subduction of the American Plate along its ridge. On the other hand, the Atlantic Ocean has been continuously expanding for the last 200 million years, and the Red Sea has started to open up. It should be noted that the closing of oceans brings the continental masses closer to each other, while opening of oceans displaces the continents away from each other.

• 700 million years ago, all landmasses were united into a single giant landmass called Pangaea I.
• About 600-500 million years ago, Pangaea I broke apart due to thermal convective currents.
• The landmasses were again united due to plate motions about 300-200 million years ago, forming Pangaea II.
• Pangaea II began to break apart during the early Jurassic period.
• North-West Africa broke away from North America and drifted away.
• The zone of sea-floor spreading continued to extend towards the north and south.
• The separation of South America and Africa occurred during the middle Cretaceous period.
• North America and Europe began to move away from each other.

The opening of the North Atlantic occurred in multiple phases. Firstly, after North America separated from Africa, Europe and Greenland broke away from Labrador during the late Cretaceous period (about 80 million years ago), forming the Labrador Sea. This newly formed sea persisted as a northern extension of the Atlantic Ocean for some time. During the Tertiary period (about 60 million years ago), the Rockall Plateau separated from Greenland.

The Labrador Sea and North Atlantic continued to expand between Europe and Greenland until the middle Miocene period, due to the continued eastward and westward movement of the European and American plates, respectively. Although the Labrador Sea stopped spreading by the middle Miocene period (about 47 million years ago), the North Atlantic continued to expand.

• Prior to the Cretaceous period, Indian Ocean did not exist.
• During the Cretaceous period, Indian plate moved towards Asiatic plate through Tethys Sea and Australian-Antarctic plates moved southward after breaking away from African plate.
• Dan Mackenzie and John Sclater have presented chronological sequence of Indian Ocean’s evolution based on study of magnetic anomalies.
• Indian plate moved northward at 18 cm per year during early Tertiary period but movement stopped during Eocene period.
• Antarctica broke away from Australia around the same time causing Pacific Ocean to shrink due to expansion of Atlantic and Indian Oceans.
• Atlantic Ocean began to open 700 million years ago due to breaking of First Pangaea when American and Africa-European plates moved in divergent directions.

• Atlantic continued to expand until 400 million years ago when it began to close, resulting in formation of Appalachian Mountains.
• Atlantic Ocean began to open up again 150 million years ago when Second Pangaea was broken into several landmasses and continues to expand due to movement of American and European plates in opposite directions.
• Atlantic Ocean has been continuously expanding for the past 200 million years while Pacific Ocean is contracting due to westward movement of the Americas
• Probable situation of continents and ocean basins in 50 million years depicted in Figure.

Evidence and Support for continental displacement, sea-floor spreading and contraction in the size of the oceans

The following examples illustrate the trends and patterns of continental displacement, sea-floor spreading, and changes in the size of oceans.

Red Sea and the Gulf of Aden

• Red Sea is an axial trough located between Africa and Arabian peninsula.
• Surveyed magnetic anomalies in this area show a stripe pattern similar to ocean basins.
• F.J. Vine calculated the spreading rate of the Red Sea in 1966 as one centimeter per year (total spreading 2 cm/year) for the past 3-4 million years.
• Alen and Morelli calculated the spreading rate of the Red Sea in 1969 as 1.1 cm/year (total spreading 2.2 cm/year).
• The rate of spreading of the Gulf of Aden has been calculated on the basis of stripped magnetic anomalies as 0.9 to 1.1 cm/year (total spreading 1.8 to 2.2 cm/year).
• The Red Sea and the Gulf of Aden are located at the junction of three plates, namely Nubian plate, Somali plate, and Arabian plate.
• Nubian and Somali plates are separated by the Ethiopian fault.
• Fig shows the location of Red Sea, Gulf of Aden, Arabian, Nubian, and Somali plates and the pole of rotation.

The Gulf of California

• The Pacific Ocean is contracting in size due to westward movement of American plates.
• There may have been a mid-oceanic ridge in the Pacific Ocean, but it has been deformed by plate movement.
• Magnetic survey of the Gulf of California shows the presence of stripped magnetic anomaly.
• East Pacific Rise (ridge) is located in the Gulf of California and continuous spreading of the gulf has occurred along the ridge for the past four million years.
• Baja, the Californian peninsula, was previously united with the mainland of North America but later broke away due to spreading of sea floor.

Implications of Plate Tectonics Theory on Geology and Earth Sciences/Evaluation

The concept of continental drift has become a reality with the validation of plate tectonics, which is widely accepted by the scientific community. However, the question of the driving force behind continental drift remains a point of debate. The majority of scientists suggest that thermal convective currents from the mantle are the probable force responsible for moving the plates or continents in different directions.

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